Urban Forest
Research
July 2002
Center for Urban Forest Research • Pacific Southwest Research Station • USDA Forest Service
Is all your rain going down the drain?
Trees are a solution
Have you ever gone outside after a
rainstorm and looked around
thinking… “where does all this
rainwater end up?” Perhaps you can
see some running down your driveway into the street. Or you have a
large puddle forming on your front
lawn. And then there is that flooded
intersection at the end of your block
because the storm drain is clogged.
Sound familiar?
Communities throughout the U.S.
are faced with this problem—too
much water and not enough places
to put it, so much of it is going
“down the drain.”
You’ve probably heard it called
another name—stormwater runoff.
And its not surprising that storm-
off is a leading cause of
impairment to nearly
40% of U.S. waterways
and led to more than
1,500 beach closings
and advisories at
coastal and Great
Lakes sites in 1998.
Urban Hydrology
Steve Lennartz
water runoff from urban, industrial,
and agricultural sources is an environmental nemesis that EPA and
other regulators have been trying to
control for more than a decade. The
agency claims that stormwater run-
As we build our
communities, considerable natural landscape
is converted to impervious surfaces such as
roads, parking lots, driveways and
buildings. Manmade drainage
systems such as sewers and storm
drains are used to improve water
movement through communities
(continued next page)
Center for Urban Forest Research
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2
How Benefits are Calculated
Our interception model accounts for water
intercepted by the tree as well as throughfall
and stem flow. Intercepted water is stored
temporarily on canopy leaf and bark surfaces.
Once the leaf is saturated, it drips from the
leaf surface and flows down the stem surface
to the ground or evaporates.
The volume of water stored in the tree
crown was calculated from the crown
projection area (area under tree dripline), leaf
area indices (LAI, the ratio of leaf surface
area to crown projection area), and water
depth on the canopy surface. Speciesspecific factors, such as crown gaps and tree
surface saturation values, influence the
amount of projected throughfall. Hourly
meteorological and rainfall data from local
sources are used for the simulations.
To estimate the value of rainfall intercepted by urban trees, we use stormwater
management control costs based on
minimum requirements for stormwater management in a particular region. For example:
In Western Washington, for a 10-acre, singlefamily residential development on permeable
soils it costs approximately $0.02779/gal to
treat and control flows stemming from a 6month, 24-hr storm event. In Fresno, the
average cost for constructing and maintaining
a typical detention/retention basin is
$121,439/ac. With a 50% probability of filling
10 times in a 20-year period, the cost of
detention/retention is $0.0077/gal. In Los
Angeles, it costs approximately $0.0183/gal
to treat sanitary waste, and we assume a
similar cost for stormwater. Runoff control for
very large events (100-year, 24-hr storm) was
omitted, as trees’ effective interception diminishes once surfaces have been saturated.
To calculate benefits, we multiply the
management cost by gallons of rainfall
intercepted after the first 0.1 inch has fallen
for each event (24-hr without rain) during the
year, depending on the region. Based on
surface detention calculations, the first 0.1
inch of rainfall seldom results in runoff. Thus,
interception is not a benefit until precipitation
exceeds this amount.
Urban Forest Research
and into drainages and natural
waterways. However, water quality
suffers when runoff carries contaminants such as oil, metals, or pesticides into streams, wetlands, lakes,
and marine waters. Management of
stormwater runoff can help reduce
this pollution and make waterways
healthy for people and fish.
Managing Stormwater Runoff
with Trees
Some of the techniques that engineers have been using to manage
stormwater runoff include infiltration, flow attenuation, retention,
detention, extended detention, and
undergrounding. See http://
www.co.ha.md.us/dpw for more
details. What you don’t see here, and
what engineers are beginning to
consider, is the use of trees to retain
water on site to slow the flow to
waterways.
Our Center’s research over the
last few years has uncovered some
very interesting facts about a tree’s
ability to retain water and how an
urban forest contributes to the
management of stormwater runoff.
Trees Retain Rainwater On
Site—Our Initial Study
In an initial study in 1998 on individual trees, we found that during a
rainfall event, precipitation is either
intercepted by leaves, branches, and
the trunk, or it falls directly through
the tree to the ground. Intercepted
water is stored temporarily on leaf
and bark surfaces. After about 10
minutes, the tree’s rainfall storage
potential gets filled, and water begins
to drip from leaf surfaces, flow down
stem and trunk surfaces to the
ground, or evaporate. We define
interception as the sum of canopy
surface water storage and
evaporation.
Results are influenced by three
factors: character and magnitude of
the rainfall event, tree species and
their architecture, and weather. Not
What is Interception?
Interception is the sum of canopy surface;
water storage on leaves, branches, and trunk
bark; and evaporation during rainfall events.
every event will produce the same
results because rainfall intensity and
duration determine the interception
process. Tree architecture, leaf and
bark surface area, and routes to
store the rainwater and control the
flow all differ by tree species.
Temperature, relative humidity,
net radiation, and wind speed
control the length of time rainfall is
retained in storage. For example, we
found that trees stored more water
during a 1-inch rainfall event that
lasted two days versus one that
lasted only two hours.
Urban Forests Make A
Significant Contribution
After investigating individual trees
we wanted to see how an entire
urban forest influenced runoff
volume. Taking the results of our
initial study of individual trees, we
created a canopy interception model
to examine the storage capacity of
the 6 million trees in Sacramento
County, California. The results show
that for just the land area covered by
trees, the county’s tree canopy intercepts 11.1% of the annual rainfall,
close to reported values for hardwood forests. However, they account
Urban Forest Research
is a publication of the Center
for Urban Forest Research,
Pacific Southwest Research
Station, USDA Forest Service. For more
information, contact the Center at the
Department of Environmental
Horticulture, University of California, 1
Shields Ave, Suite 1103, Davis, CA 956168587. (530) 752-7636
USDA is an equal opportunity provider and
employer, and prohibits discrimination in all
programs and activities.
Editor: Laurie Litman, InfoWright
July 2002
Fact Sheet #4: Control Stormwater Runoff with Trees
Points to remember
RAINFALL INTERCEPTION is influenced
by:
Intensity and duration of the
rainfall event
Tree species—deciduous,
broadleaf evergreen, or conifer
Tree architecture—size, number
of leaves, and arrangement of
leaves and branches
Weather—temperature, relative
humidity, net solar radiation,
and wind speed
TREES STORE MORE WATER during a 1inch rainfall event that lasts two
days versus one that lasts only two
hours. Therefore:
As compared to flood events,
small storms are responsible for
most of the annual pollutant
loading of receiving waters
Trees are most effective in
intercepting rainfall during small
events
Urban forests are likely to
produce more benefits through
water quality protection than
flood control.
ONE OF OUR STUDIES FOUND that a
typical medium-sized tree can
intercept as much as 2380 gallons of
rainfall per year.
BROADLEAF EVERGREENS AND CONIFERS
intercept more rainfall than
deciduous species where winter
rainfall patterns prevail.
street
porous pavers
grass pavers
sidewalk
Renderings by Alan A. Loomis, http://www.deliriousla.net
Redesign streets where trees work in combination with grass and porous
pavers to retain water on site.
TREES WORK IN COMBINATION with other
stormwater controls to produce a
comprehensive solution to rainfall
interception, runoff and landscape
water use:
Backyard cisterns capture roof
runoff, and provide supplemental irrigation
Swales hold overflow
Bermed lawn-area retention
basins facilitate infiltration
Grates/drywells capture
driveway runoff
STRATEGIES TO ENHANCE the urban
forest and improve the control of
stormwater runoff:
Plant more trees in appropriate
places
Improve the maintenance of
existing trees
Plant species with a higher rate
of growth where appropriate
Plant species with architectural
features that maximize
interception
Match trees (deciduous,
evergreen) to rainfall patterns
Plant trees in groves where
possible
Plant low water-use species
Plant broadleaf evergreens
where appropriate and avoid
south-facing windows
Use native plants, which, once
established, can easily withstand
summer dry seasons and reduce
the need for supplemental
irrigation.
In Oakland, California, the continuous tree
canopy is estimated to intercept 4 inches of
rain over one acre in a typical year— about
108,000 gallons.
NOTE: In looking for solutions to stormwater runoff it is important to consider an integrated approach that uses other water
conservation, water retention, flood management, and pollution control strategies. Community solutions include but are not
limited to: porous pavement, vegetated swales and filter strips, recharge areas under parking lots, holding tanks and cisterns
under playfields, surface area holding ponds, turf grass filters, and riparian retention and treatment areas. For more
information on these solutions see the TreePeople website at http://www.treepeople.org/trees/charrette.htm, and their book,
Second Nature: Adapting Los Angeles' Landscape for Sustainable Living, edited by Patrick Condon and Stacy Moriarity.
July 2002
Urban Forest Research
References: Control Stormwater Runoff with Trees
For more information, refer to the following publications:
Chang, G.; J. Parrish; and C. Souer.
1990. The first flush of runoff and its
effect on control structure design.
Environmental Resource
Management Division. Department
of Environmental and Conservation
Services. City of Austin, Austin, TX.
36 pp.
Claytor, R. A.; and T. R. Schueler.
1996. Design of stormwater filtering
systems. The Center for Watershed
Protection, Silver Spring, MD.
McPherson, E. G. 1998. Structure
and sustainability of Sacramento’s
urban forest. J. Arbor. 24(4): 174190.
McPherson, E. G., et. al. 2000. Tree
guidelines for coastal Southern
California communities. Local
Government Commission,
Sacramento, CA. 97p.
McPherson, E. G., et. al. 1999
Benefit-cost anlysis of Modesto’s
municipal urban forest. J. Arbor.
25(5): 235-248.
Xiao, Q. F., et. al. 2000. Winter
rainfall interception by two mature
open-grown trees in Davis,
California. Hydrol. Process. 14: 763784.
Xiao, Q. F., et. al. 2000. A new
approach to modeling tree rainfall
interception. J. of Geophysical
Research. 105(D23): 29, 173-29,
188.
Xiao, Q. F., et. al. 1998. Rainfall
interception by Sacramento’s urban
forest. J. Arbor. 24(4): 235-244.
Trees protect water and soil
resources.
A healthy urban forest can
reduce the amount of runoff and
pollutant loading in receiving
waters in four primary ways:
1) Through evapotranspiration,
trees draw moisture from the soil
ground surface, thereby
increasing soil water storage
potential.
2) Leaves, branch surfaces, and
trunk bark intercept and store
rainfall, thereby reducing runoff
volumes and delaying the onset
of peak flows.
3) Root growth and
decomposition increase the
capacity and rate of soil
infiltration by rainfall and reduce
overland flow.
4) Tree canopies reduce soil
erosion by diminishing the
impact of raindrops on barren
surfaces.
Urban forests can dispose of
waste water
Incorporate stormwater treatment into street design by
adding trees and swales. From Green Neighborhoods, a publication
of NeighborhoodLAB, http://neighborhood.uoregon.edu/.
Visit our newly redesigned website at
http://cufr.ucdavis.edu
Urban forests can provide other
hydrologic benefits. For example,
irrigated tree plantations or
nurseries can be a safe and
productive means of wastewater
treatment and disposal. Reused
wastewater can recharge
aquifers, reduce stormwater
treatment loads, and create
income through sales of nursery
or wood products. Recycling
urban wastewater into greenspace areas can be an economical
means of treatment and disposal,
while at the same time providing
other environmental benefits.
This fact sheet is provided for you to copy and distribute. Please credit the Center for Urban Forest Research, Pacific Southwest Research
Station, USDA Forest Service, Davis, California. July 2002.
Urban Forest Research
July 2002
3
Rainfall Interception Per Event (greater than 1/4”)
How much rain are we talking about?
Species
One inch of rain over one acre is about
27,000 gallons.
Dbh (in)
Crown projection (sq ft)*
Interception (gal)
Summer Winter Total
Large
Plane Tree1
Camphor2
16
22
1314
1227
99
62
32
62
131
123
Medium
Chinese Pistache1
Jacaranda2
10
11
800
608
45
35
20
15
65
50
4
9
123
600
5
15
3
15
8
31
Small
Crape Myrtle1
Podocarpus2
*Crown projection = Area under the drip line.
Modesto, CA, winter rainfall pattern 2 Santa Monica, CA, winter rainfall pattern
1
for only 1.1% of the interception
over the entire region because of the
region’s relatively low tree density
and pattern of winter rainfall when
deciduous trees are leafless.
The mix of tree species and their
sizes influence interception. In
Sacramento County, evergreen trees
play the most important role in
interception because most precipitation occurs in winter. Large trees
with evergreen foliage contribute to
greater interception than smaller,
deciduous trees. In many climates
with summer precipitation, deciduous trees make a substantial contribution to rainfall interception.
Planting more trees and improv-
Shade yields less water use at power
plants
Power plants consume water in the process of
producing electricity. For example, coal-fired
plants use about 0.6 gal/kWh of electricity
provided. Trees that reduce the demand for
electricity can also reduce water consumed at
the power plant (McPherson et al. 1993).
Precious surface water resources are
preserved, and thermal pollution of rivers is
reduced.
July 2002
ing maintenance of existing trees are
important strategies that will help
Sacramento, as well as many other
communities, reduce the volume of
stormwater runoff.
Rainfall and Tree Effectiveness
One effect that became clear in the
Sacramento study was that urban
forests become increasingly less
effective at reducing stormwater
runoff as the amount of precipitation
per storm increases. Although trees
reduce runoff, they may not be very
effective for flood control.
Floods usually occur during
major storm events, well after
canopy storage has been exceeded.
However, by substantially reducing
the amount of runoff during less
extreme events, urban forests can
protect water quality. Small storms,
for which urban forest interception
is greatest, are responsible for most
annual pollutant loading. Infrequently occurring large storms usually
produce the greatest flooding
damage, and although they may
contain significant pollutant loads,
their contribution to the annual
average pollutant load is quite small
(Chang et al. 1990).
Also, because of the infrequent
occurrence of large storms, receiving
waters have relatively long periods of
recovery between events (Claytor
and Schueler 1996). Therefore,
urban forests are likely to produce
more benefits through water quality
protection than flood control.
Taking the Next Step
Since trees are only a partial solution to managing stormwater runoff,
the next step we’ve taken is to
investigate other techniques to
create a comprehensive approach
that communities can take to keep
most of their water from going down
the drain. In collaboration with
TreePeople in Los Angeles, we have
selected two single-family sites in LA
to evaluate four stormwater management techniques—cisterns, retention/detention basins, swales, and a
driveway grate and drywell.
The sites we chose are adjacent
lots with the same dimensions (50 ft
wide by 150 ft deep). The techniques were installed on the treatment site only; the control site was
left unmodified. The 3000-gallon
cistern receives and stores filtered
roof runoff and functions like a minireservoir for both runoff control and
summer irrigation. The retention/
detention basins (front and back
lawns) retain roof runoff. The roof
runoff infiltrates, evaporates, or
overflows to the swale or to the
street. The grate at the end of the
driveway drains runoff into a drywell
under the lawn, and the overflow is
recharged in the retention/detention
basin.
Preliminary Results
We have found that all stormwater
runoff has been retained on the
(continued)
Urban Forest Research
4
treatment site. We also found that
the cistern storage provides about
10% of the annual water to irrigate
the landscape. At the control site,
runoff from half the roof and the
entire driveway was discharged to
the street.
Based on soil property measurements, both sites are situated on
deep, sandy soil. The infiltration rate
of these soils is greater than a 50year flood event. What we don’t
know about this highly permeable
soil: Does this on-site stormwater
retention cause groundwater contamination? Are we just transferring
the surface water problem to the
groundwater?
We also don’t know the effectiveIn Modesto, CA, each street and park tree
was estimated to reduce stormwater runoff by
845 gallons annually, with a benefit valued at
$7 per tree (McPherson et al. 1999). A typical
medium-sized tree in coastal southern
California was estimated to intercept 2,380
gallons annually, a $5 per tree benefit
(McPherson et al. 2000). These studies
showed that broadleaf evergreens and
conifers intercept more rainfall than deciduous
species where winter rainfall patterns prevail.
ness of on-site runoff retention in
different geological settings, soil
types, and landscape designs. These
questions require further study.
The Future
The goal of this project is to examine
and model stormwater management
techniques at the residential scale.
The hydrologic and ecologic performance of this demonstration site
will be monitored over the long term
to help determine the problems and
opportunities associated with
treating each Los Angeles-area site
as a mini-watershed.
By combining the model of these
traditional techniques with our tree
model, we will be able to more
completely demonstrate the impact
that a comprehensive solution will
have on rainfall interception, runoff,
and landscape water use. All of the
rain does not have to go down the
drain. Most, if not all, can be
retained on site with a portion used
for summertime irrigation.
—Jim Geiger
This research was a partnership
between the Forest Service and
University of California, Davis,
performed by Dr. Qingfu Xiao, research affiliate with the Department
of Land, Air, & Water Resources.
Welcome aboard
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Upcoming Presentations
August 27, 2002
Keynote Address: Municipal forestry
benefit-cost analysis: a comparison
of Modesto and Santa Monica, CA,
by Greg McPherson. European
Regional Conference of IUFRO,
Copenhagen, Denmark.
September 13, 2002
A practical approach to assessing
structure, function, and value of
street tree populations in small
communities, by Scott Maco. 2002
California Urban Forest Conference,
Visalia, CA
September 13, 2002
Influencing local decision makers to
invest in the urban forest, by Jim
Geiger. 2002 California Urban Forest
Conference, Visalia, CA
September 14, 2002
Planning for energy savings—using
trees to reduce costs, cool
communities, cool parking lots,
schools, etc., by Jim Simpson. 2002
California Urban Forest Conference,
Visalia, CA
September 25, 2002
Preserving the urban forest: creative
sidewalk repair and replacement, by
Greg McPherson with City of Los
Angeles Street Tree Division.
American Public Works Association
Congress, Kansas City, MO.
September 27, 2002
Costs and benefits of urban trees in
relation to smart growth, by Greg
McPherson. Community Forestry at
its Best, Nebraska City, NB.
Address _________________________________________________________________
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Comments or suggestions? __________________________________________________
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Send to Center for Urban Forest Research, Pacific Southwest Research Station, USDA
Forest Service, c/o Department of Environmental Horticulture, University of California,
1 Shields Avenue, Suite 1103, Davis, CA 95616-8587 or contact jgeiger@fs.fed.us.
Urban Forest Research
Find urban forestry
resources and
information at our
redesigned website
http://cufr.ucdavis.edu/
July 2002